How to "Reverse Engineer" mesh created out of smooth curves with a missing chunk?
You can fill holes in curved surfaces like this either with extruding with snapping set to Vertex or using Mesh:F2 addon.

Using F2

After enabling add-on, select vertex in the corner of the hole and hover mouse cursor over hole (mouse position matters regarding to where face is created if there are several possible ways).
After that press F to fill the face. Continue until finished with other vertices.
Split uneven parts of hole to make it of square / rectangular form. Once it is use Ctrl+F > Grid Fill and choose options to have clean topology.



See more about filling faces with F2 addon: Make quad face from where two edges will meet?

Extruding and snapping 

The same could be done with extruding with snapping turned on and set to Vertex as element to snap to. Fill the surface until the hole is rectangular and then use Grid Fill.